Mass spectrometry has proven to be an effective analytical technique for identifying unknown compounds and determining the precise mass of known compounds. Advantageously, compounds can be detected or analyzed in minute quantities allowing compounds to be identified at very low concentrations in chemically complex mixtures. Not surprisingly, mass spectrometry has found practical application in medicine, pharmacology, food sciences, semi-conductor manufacturing, environmental sciences, security, and many other fields.
A typical mass spectrometer includes an ion source that ionizes particles of interest. The ions are passed to an analyser region, where they are separated according to their mass (m)-to-charge (z) ratios (m/z). The separated ions are detected at a detector. A signal from the detector is sent to a computing or similar device where the m/z ratios are stored together with their relative abundance for presentation in the format of a m/z spectrum.
Typical ion sources are exemplified in “Ionization Methods in Organic Mass Spectrometry”, Alison E. Ashcroft, The Royal Society of Chemistry, UK, 1997; and the references cited therein. Conventional ion sources may create ions by atmospheric pressure chemical ionisation (APCI); chemical ionisation (CI); electron impact (EI); electrospray ionisation (ESI); fast atom bombardment (FAB); field desorption/field ionisation (FD/FI); matrix assisted laser desorption ionisation (MALDI); or thermospray ionization (TSP).
Ionized particles may be separated by quadrupoles, time-of-flight (TOF) analysers, magnetic sectors, Fourier transform and ion traps.
The ability to analyse minute quantities requires high sensitivity. High sensitivity is obtained by high transmission of analyte ions, and low transmission of non-analyte ions and particles, known as chemical background.
An ion guide guides ionized particles between the ion source and the analyser/detector. The primary role of the ion guide is to transport the ions toward the low pressure analyser region of the spectrometer. Many known mass spectrometers produce ionized particles at high pressure, and require multiple stages of pumping with multiple pressure regions in order to reduce the pressure of the analyser region in a cost-effective manner. Typically, an associated ion guide transports ions through these various pressure regions.
One approach to obtain high sensitivity is to use large entrance apertures, and smaller exit apertures, to transport ions from regions of higher pressure to lower pressure. Vacuum pumps and multiple pumping stages reduce the pressure in a cost-effective way. Thus, the number of ions entering the analyser region is increased, while the total gas load along various pressure stages is decreased. Often the ion guide includes several such stages of accepting and emitting the ions, as the beam is transported through various vacuum regions and into the analyser.
For high sensitivity low ion losses at each stage are desirable. Therefore it is advantageous to reduce the radius of the ion beam, to produce a small beam diameter at the exit, from a large initial beam diameter at the entrance aperture. That is, the maximum radial excursion of a set of individual ions in the ion beam is reduced as the ions traverse axially along the ion path before the exit, thereby concentrating the ion beam. Generally, the more concentrated the beam entering the analyser, the higher the desired ion flux and the greater the overall sensitivity of the mass spectrometer.
One typical guide includes multiple parallel rods, with nearly equal size entrance and exit apertures. Typically four, six, eight, or more, rods, are arranged in quadrupole, hexapole, or the like. A DC voltage with a superimposed high frequency RF voltage is applied to the rods. The frequency and amplitude of the applied voltage is the same for all rods, but the phases of the high frequency voltages of adjacent rod electrodes are reversed. Another conventional RF ion guide is formed as a set of parallel rings or plates with apertures. Again, RF and DC voltages are applied to the rings or plates.
These conventional ion guides provide additional functionality at moderate pressure, such as ion mobility separation by the application of an axial drift field (as, for example, G. Javahery and B. Thomson, J. Am. Soc. Mass. Spectrom. 8, 692 (1997)); and ion trapping (Raymond E. March, John F. J. Todd, Practical Aspects of Ion Trap Mass Spectrometry: Volume 2: Ion Trap Instrumentation, CRC Press Boca Raton, Fla. 1995). Further, quadrupole ion guides allow for mass-to-charge selective excitation and ejection by use of resonant excitation methods.
Commonly, in RF ion guides at moderate pressures, collisions of ions with background gas cause some reduction of the radial amplitude, and help to concentrate the ion beam near the exit. (as for example detailed in U.S. Pat. No. 4,963,736; and R. E. March and J. F. J. Todd (Eds.), 1995, Practical Aspects of Ion Trap Mass Spectrometry: Fundamentals, Modern Mass Spectrometry Series, vol. 1. (Boca Raton, Fla.: CRC Press)).
However, it is not always possible to efficiently concentrate an ion beam at the entrance or exit of a conventional RF ion guide. For example, as the ion and gas exit a high pressure region into a lower pressure region, through a large aperture, the ion beam may be entrained in a flow of high density gas. The ions in the high density gas cannot be readily guided or concentrated. Ions may be scattered in the high density gas, and lost to the rod electrodes. At the exit, the degree to which the ion beam may be concentrated is limited at least partly by the pressure and RF voltage, in practice for electrical reasons such as discharge and creep.
Although some existing RF ion guides do further concentrate the ion beam, they have disadvantages due to their geometries. These ion guides include one or more sets of plates or discs, with variable apertures, separated by gaps, with unequal size entrance and exit apertures. The geometries typically result in distortions of the electric field that reduce the sensitivity of the mass spectrometer. This problem can be acute in ion guides that accumulate ions in guided ion beams. Typically, stored ions are passed back and forth through the ion guide prior to ejection, sometimes many times. Poorly defined electric fields can induce losses in transmission as ions undergo repeated passes, causing the ions to escape from or collide with the guide. Similarly ion separation on the basis of mobility is less effective due to broadening of the ion separation time and diffusion losses. Finally, these ion guides do not preserve ion motion by maintaining or incrementally varying the ions' oscillatory frequency as they travel through the guide, reducing mass-to-charge selective excitation methods.
Thus, there exists a need for an ion guide and method that reduces the radius of travel of the ion beam about a guide axis, and also combines some of the benefits with few of the disadvantages associated with the conventional ion guides and techniques. Such a device and method would improve the sensitivity and usefulness of the mass spectrometer and have wide applicability and higher sensitivity than conventional ion guides and methods that are commonly available.